Method of making male sterile petunia plants by transformation with a nucleic acid encoding a zinc finger transcriptional factor

ABSTRACT

A method is provided for producing a male sterile plant of the family Solanaceae by utilizing a plant expression cassette including a nucleic acid which is DNA encoding zinc finger transcription factors (ZPT2-5) derived from  Petunia  and a promoter operatively linked to the nucleic acid.

This application claims priority to application no. PCT/JP99/06467 filed 19 Nov. 1999, now publication no. WO 01/37643 published 31 May 2001 which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to genes which are expressed specifically in the pollen producing tissues of stamens and use of the same. More particularly, the present invention relates to the genes for zinc finger transcription factors (ZPT2-5, ZPT3-1 and ZPT4-1) derived from Petunia, are expressed specifically in microspores, and use of the same.

BACKGROUND ART

Pollen fertility causes problems in various aspects of agriculture and horticulture. For example, in the case of mating for cross breeding, self-pollination has to be avoided by castration (removal of stamens) which requires enormous effort. In the seed and seedling industry, there is a demand for a trait of lack of pollen fertility from the standpoint of commercially protecting excellent breeds obtained by cross breeding. To meet such a demand, a technique for controlling pollen fertility (pollination control) has been strongly required. Conventionally, for particular crops, lines of cytoplasmic male sterility have been used for cross breeding, and some success has been achieved. However, the cytoplasmic sterility trait is often accompanied by undesired side effects, such as a reduction in disease resistance and the like. There are further problems, such as that the trait is unstable, that it is difficult to mass-produce the seeds, and the like. A method for reducing the fertility by treating with a chemical agent(s) has been studied, but safety evaluation and elucidation of the mechanism of this method have not been fully done and thus such a method is not yet in actual use. Therefore, there is a demand for an excellent male sterilization technique using genetic engineering.

Pollen is the male gametophyte of spermatophyte. The development of pollen which proceeds while pollen is surrounded by an anther as a supporting tissue is divided into the following stages: the tetrad stage immediately after the meiosis of microsporogenous cells (pollen mother cells); the release stage during which microspores are released from the tetrad; the uninucleate stage characterized by the enlargement and vacuolation of pollen cells, the mitotic stage giving rise to the differentiation into vegetative and generative cells by mitosis; and the subsequent binucleate stage. After these stages, the anther finally dehisces and matured pollen grains are released. Therefore, it can be said that the microspore is one of target tissues which are most suitable for artificial control in order to inhibit the development of pollen and eliminates pollen fertility.

As described above, great expectations are placed on male sterilization techniques using genetic engineering. Particularly, if a gene which is expressed specifically in the direct precursor of a pollen cell, such as a microspore, can be utilized, it is considered to be highly likely that male sterilization can be achieved without conferring undesired traits to plants. Several examples of promoters specific to various stamen tissues and gene constructs for male sterilization comprising the promoter have been reported (Shivanna and Sawhney Ed., Pollen biotechnology for crop production and improvement (Cambridge University Press), pp. 237–257, 1997). However, there has been continuously a demand for a novel gene useful for control of pollen fertility, which has high tissue and temporal specificities of expression.

Recently, the inventors of the present application specified the cDNA sequences of novel transcription factors derived from Petunia, i.e., seven zinc finger (ZF) transcription factors including PEThy ZPT2-5, PEThy ZPT3-1, and PEThy ZPT4-1 (hereinafter abbreviated as ZPT2-5, ZPT3-1, and ZPT4-1, respectively). And the inventors reported that Northern blot analysis indicates that each transcription factor transiently expresses in an anther-specific manner in a different stage of the development of the anther (Kobayashi et al., Plant J., 13:571, 1998). However, the physiological function and action of these transcription factors in plants, and the precise expression sites and the expression controlling mechanism of the genes encoding the transcription factors have been not clarified.

DISCLOSURE OF THE INVENTION

The objective of the present invention is to provide a genetic engineering technique using a pollen-specific gene which is useful for modification of a plant trait, representatively male sterility.

The present inventors reintroduced genes encoding anther-specific transcription factors (ZPT2-5, ZPT3-1 and ZPT4-1), which had been previously isolated from Petunia, into Petunia. As a result, it was found that the normal development of pollen was inhibited, so that pollen fertility was significantly reduced (ZPT2-5 and ZPT4-1), or substantially eliminated (ZPT3-1). Further, the inventors isolated upstream regions of the ZPT3-1 and ZPT4-1 genomic genes, respectively, and studied the tissue specificity of the promoter activity. As a result, it was found that the promoter activity is expressed in microspores from the uninucleate stage to the binucleate stage in a tissue and temporal-specific manner. The present invention was completed based on these findings.

According to a first aspect of the present invention, a method for producing a male sterile plant comprises the steps of providing a plant expression cassette including: a nucleic acid being any of (i) DNA having a sequence from position 1 to position 777 of a base sequence indicated by SEQ ID NO: 1, (ii) DNA hybridizing the DNA having the base sequence (i) under stringent conditions and encoding a transcription factor controlling the development of pollen, and (iii) a DNA fragment of (i) or (ii); and a promoter operatively linked to the nucleic acid, providing plant cells having an endogenous transcription factor controlling the development of pollen, wherein a gene encoding the endogenous transcription factor hybridizes the nucleic acid under stringent conditions, introducing the expression cassette into the plant cells, regenerating the plant cells, into which the expression cassette has been introduced, to plants; and screening the regenerated plants for one in which the nucleic acid is expressed so that expression of the endogenous transcription factor is suppressed.

According to a second aspect of the present invention, a method for producing a male sterile plant comprises the steps of providing a plant expression cassette including: a nucleic acid being any of (i′) DNA having a sequence from position 1 to position 1640 of a base sequence indicated by SEQ ID NO: 3, (ii′) DNA hybridizing the DNA having the base sequence (i′) under stringent conditions and encoding a transcription factor controlling the development of pollen, and (iii′) a DNA fragment of (i′) or (ii′); and a promoter operatively linked to the nucleic acid, providing plant cells having an endogenous transcription factor controlling the development of pollen, wherein a gene encoding the endogenous transcription factor hybridizes the nucleic acid under stringent conditions, introducing the expression cassette into the plant cells, regenerating the plant cells, into which the expression cassette has been introduced, to plants; and screening the regenerated plants for one in which the nucleic acid is expressed so that expression of the endogenous transcription factor is suppressed.

According to a third aspect of the present invention, a method for producing a male sterile plant comprises the steps of providing a plant expression cassette including: a nucleic acid being any of (i″) DNA having a sequence from position 1 to position 1948 of a base sequence indicated by SEQ ID NO: 5, (ii″) DNA hybridizing the DNA having the base sequence (i″) under stringent conditions and encoding a transcription factor controlling the development of pollen, and (iii″) a DNA fragment of (i″) or (ii″); and a promoter operatively linked to the nucleic acid, providing plant cells having an endogenous transcription factor controlling the development of pollen, wherein a gene encoding the endogenous transcription factor hybridizes the nucleic acid under stringent conditions, introducing the expression cassette into the plant cells, regenerating the plant cells, into which the expression cassette has been introduced, to plants; and screening the regenerated plants for one in which the nucleic acid is expressed so that expression of the endogenous transcription factor is suppressed.

It should be noted that the DNAs of (ii), (ii′) and (ii″) each do not include (iv) DNA hybridizing DNA having a sequence from position 1 to position 1886 of abase sequence indicated by SEQ ID NO: 13 under stringent conditions and encoding a transcription factor controlling the development of pollen. The base sequence indicated by SEQ ID NO: 13 is a cDNA sequence encoding another transcription factor ZPT3-2 isolated from Petunia (Kobayashi et al. above).

The method according to the first through third aspects of the present invention is utilized as a method for conferring male sterility to a plant.

In one embodiment of the first through third aspects, the nucleic acid is linked in a forward direction with respect to the promoter, and may be transcribed in a sense direction in cells of the plant.

In one embodiment of the first through third aspects, the nucleic acid is linked in a reverse direction with respect to the promoter, and may be transcribed in a antisense direction in cells of the plant.

In one embodiment of the first through third aspects, the plant is dicotyledon. The dicotyledon is preferably of the family Solanaceae, and more preferably of the genus Petunia.

In one embodiment of the first through third aspects, the expression cassette is incorporated into a plant expression vector.

According to the first through third aspects of the present invention, a male sterile plant produced by a method according to any of the above-described methods is also provided.

According to a fourth aspect of the present invention, a method for producing a plant having a modified trait comprises the steps of providing a plant expression cassette including: a promoter including any of (a′) DNA having a sequence from position 1 to position 2624 of a base sequence indicated by SEQ ID NO: 7 and (b′) DNA having a part of the sequence of (a′) and exhibiting promoter activity specific to microspores; and a heterologous gene operatively linked to the promoter, introducing the expression cassette into plant cells, and regenerating the plant cells, into which the expression cassette has been introduced, to plants.

According to a fifth aspect of the present invention, a method for producing a plant having a modified trait comprises the steps of providing a plant expression cassette including: a promoter including any of (a″) DNA having a sequence from position 1 to position 3631 of a base sequence indicated by SEQ ID NO: 8 and (b″) DNA having a part of the sequence of (a″) and exhibiting promoter activity specific to microspores and optionally the dehiscence tissue of an anther; and a heterologous gene operatively linked to the promoter, introducing the expression cassette into plant cells, and regenerating the plant cells, into which the expression cassette has been introduced, to plants.

In one embodiment of the fourth and fifth aspects, the trait is fertility, and the plant having a modified trait is a male sterile plant. Therefore, the method of the present invention may be utilized as a method for conferring male sterility to a plant.

In one embodiment of the fourth and fifth aspects, the trait is compatibility, and the plant having a modified trait is a self-incompatibile plant. Therefore, the method of the present invention may be utilized as a method for conferring self-incompatibility to a plant.

In one embodiment of the fourth and fifth aspects, the plant is dicotyledon. The dicotyledon is preferably of the family Solanaceae, and more preferably of the genus Petunia.

In one embodiment of the fourth and fifth aspects, the expression cassette is incorporated into a plant expression vector.

In one embodiment of the fourth and fifth aspects, a trait-modified plant produced by a method according to any of the above-described methods is provided.

According to a sixth aspect of the present invention, a promoter comprises DNA of the following (I′) or (II′): (I′) DNA having a sequence from position 1 to position 2624 of a base sequence indicated by SEQ ID NO: 7; and (II′) DNA having a part of the sequence of (I′) and exhibiting promoter activity specific to microspores.

According to a seventh aspect of the present invention, a promoter comprises DNA of the following (I″) or (II″): (I″) DNA having a sequence from position 1 to position 3631 of a base sequence indicated by SEQ ID NO: 8; and (II″) DNA having a part of the sequence of (I″) and exhibiting promoter activity specific to microspores and optionally the dehiscence tissue of an anther.

According to an eighth aspect of the present invention, a plant expression cassette useful for conferring male sterility to a plant, comprising any of the above-described microspore-specific promoters and a heterologous gene operatively linked to the promoter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a cDNA sequence of a gene encoding ZPT2-5 (herein also simply referred to as “ZPT2-5 gene”) (SEQ ID NO: 1) and the corresponding amino acid sequence (SEQ ID NO: 2). Two zinc finger motifs and a DLNL sequence (amino acids from position 145 to position 155) are underlined.

FIG. 2 is a diagram showing a cDNA sequence of a gene encoding ZPT3-1 (herein also simply referred to as “ZPT3-1 gene”) (SEQ ID NO: 3) and the corresponding amino acid sequence (SEQ ID NO: 4). Three zinc finger motifs and a DLNL sequence (amino acids from position 408 to position 417) are underlined.

FIG. 3 is a diagram showing a cDNA sequence of a gene encoding ZPT4-1 (herein also simply referred to as “ZPT4-1 gene”) (SEQ ID NO: 5) and the corresponding amino acid sequence (SEQ ID NO: 6). Four zinc finger motifs and a DLNL sequence (amino acids from position 438 to position 449) are underlined.

FIG. 4 is a schematic diagram showing structures of plant expression vectors used for expression of each cDNA sequence of ZPT2-5, ZPT3-1 and ZPT4-1 (pBIN-35S-ZPT2-5, pBIN-35S-ZPT3-1 and pBIN-35S-ZPT4-1).

FIG. 5 is a diagram showing an upstream sequence of the coding region of the ZPT3-1 gene (SEQ ID NO: 7). The transcription initiation site is indicated by a thick arrow (position 2567). The translation initiating codon (ATG) is indicated by a thick underline.

FIG. 6 is a diagram showing an upstream sequence of the coding region of the ZPT4-1 gene (SEQ ID NO: 8). The transcription initiation site is indicated by a thick arrow (position 3503). The translation initiating codon (ATG) is indicated by a thick underline.

FIG. 7 is a schematic diagram showing structures of plant expression vectors for analyzing promoters for the ZPT3-1 and ZPT4-1 genes (pBIN-ZPT3-1-GUS and pBIN-ZPT3-1-GUS).

FIG. 8 shows photographs indicating the forms of organisms, i.e., the pollen of a wild type Petunia and the pollen of a Petunia into which pBIN-35 S-ZPT2-5 was introduced (a transformant in which cosuppression occurred) (the magnification is 400 times). FIGS. 8( a) through (d) are of the wild-type Petunia and FIGS. 8( e) through (h) are of the cosuppressed transformed Petunia, each of which shows the pollen of a bud at a different development stage. All the pollen were stained by a commonly used method using DAPI (4′,6-diamidino-2-phenylindole dihydrochloride n-hydrate).

FIG. 9 shows photographs indicating the forms of organisms, i.e., the pollen of a wild type Petunia and the pollen of a Petunia into which pBIN-35S-ZPT3-1 was introduced (the magnification is 700 times). FIGS. 9( a) and (c) are of the wild-type Petunia and FIGS. 9( b) and (d) are of the transformed Petunia, each of which shows the pollen at the tetrad stage and the microspore stage, respectively. The pollen of the tetrad stage and the pollen of the microspore stage were stained by a commonly used method using DAPI and safranin, respectively. The pollen of the Petunia into which pBIN-35S— ZPT4-1 was introduced showed substantially the same form as FIGS. 9( b) and 9(d).

FIG. 10 shows photographs showing the forms of organisms, i.e., GUS-stained floral organs of Petunia into which pBIN-ZPT3-1-GUS and pBIN-ZPT4-1-GUS were introduced. Each photograph was taken of a flower (bud) whose anther is in the uninucleate stage. FIGS. 10( a) and (d) show the appearances of bud at the actual size. FIGS. 10( b) and (e) show the cross-sectional views of an anther at a low magnification (40 times). FIGS. 10( c) and (f) show the cross-sectional views of microspores (FIG. 10( c); the magnification is 700 times) and the dehiscence tissues and the surrounding vicinity of the anther (FIG. 10( f); the magnification is 200 times) at high magnification.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

(Transcription factors derived from ZPT2-5, ZPT3-1 and ZPT4-1 genes)

A nucleic acid, which is useful in a method for producing male sterile plants according to first to third aspects of the present invention, is any one of the following DNAs:

-   -   (i) DNA having a sequence from position 1 to position 777 of a         base sequence indicated by SEQ ID NO: 1;     -   (i′) DNA having a sequence from position 1 to position 1640 of a         base sequence indicated by SEQ ID NO: 3:     -   (i″) DNA having a sequence from position 1 to position 1948 of a         base sequence indicated by SEQ ID NO: 5,     -   DNA which hybridizes to the DNA having any of the base         sequences (i) to (i″) under stringent conditions, and encodes a         transcription factor which controls the development of pollen         (i.e., (ii), (ii′) or (ii″)); or     -   DNA which is a fragment of any of the above-described DNAs         (i.e., (iii), (iii′) or (iii″)).

The above-described nucleic acid of the present invention is preferably DNA of (i), (i′) or (i″), i.e., DNA encoding ZPT2-5, ZPT3-1 or ZPT4-1, or a fragment thereof, and more preferably DNA of (i), (i′) or (i″).

In the present specification, “transcription factor” refers to a protein for controlling the synthesis of mRNA by binding to DNA in the regulatory region of a gene. It is known that a certain type of transcription factor has a highly conserved amino acid sequence called a zinc finger (ZF) motif in the DNA binding domain. ZPT2-5 is a zinc finger (ZF) protein of the Cys2/His2 type (EPF family), which is a transcription factor which includes two ZF motifs in the full-length amino acid sequence consisting of 176 amino acids, and further, a hydrophobic region called a DLNL sequence. Similarly, ZPT3-1 is a ZF protein of the EPF family, which is a transcription factor which includes three ZF motifs in the full-length amino acid sequence consisting of 437 amino acids, and further, a DLNL sequence. Similarly, ZPT4-1 is a ZF protein of the EPF family, which is a transcription factor which includes four ZF motifs in the full-length amino acid sequence consisting of 474 amino acids, and further, a DLNL sequence. For any of the above-described transcription factors, see Kobayashi et al. (above). cDNA sequences (SEQ ID NO: 1, 3 and 5) encoding ZPT2-5, ZPT3-1 and ZPT4-1, respectively, are shown in FIGS. 1, 2 and 3 along with corresponding putative amino acid sequences (SEQ ID NO: 2, 4 and 6).

In the present specification, “fragment” of a nucleic acid or DNA refers to a fragment which can inhibit the expression of an endogenous transcription factor in a plant when the fragment is introduced into the plant and expressed in an appropriate manner. This fragment is selected from regions of DNAs of the above-described (i), (i′), (i″), (ii), (ii′) or (ii″) other than the regions encoding the zinc finger motifs in the DNAs. The fragment has a length of at least about 40 bases or more, preferably about 50 bases or more, more preferably about 70 bases or more, and even more preferably about 100 bases or more.

In the present specification, “stringent conditions” for hybridization are intended as conditions sufficient for the formation of a double-strand oligonucleotide of a particular base sequence (e.g., DNA encoding ZPT2-5, ZPT3-1 or ZPT4-1 derived from Petunia) and another base sequence having a high level of homology with the particular base sequence (e.g., DNA encoding a homolog of ZPT2-5, ZPT3-1 or ZPT4-1 which is present in a plant other than Petunia). A representative example of the stringent conditions applied to the present invention are the following: hybridization is conducted in a solution containing 1M NaCl, 1% SDS, 10% dextran sulfate, ³²P-labeled probe DNA (1×10⁷ cpm) and 50 μg/ml salmon sperm DNA at 60° C. for 16 hours, followed by washing twice with 2×SSC/1% SDS at 60° C. for 30 minutes.

In the present invention, a degenerate primer pair corresponding to a conserved region of an amino acid sequence encoded by the gene of a known transcription factor may be used in order to isolate DNAs encoding ZPT2-5, ZPT3-1 and ZPT4-1, and DNA encoding a transcription factor which hybridizes these DNAs under stringent conditions to inhibit the development of pollen. PCR is conducted using this primer pair with cDNA or genomic DNA of a plant as a template, thereafter, the resultant amplified DNA fragment is used as a probe so that the cDNA or genomic library of the same plant can be screened. As an example of such a primer pair, a combination of 5′-CARGCNYTNGGNGGNCAY-3′ (SEQ ID NO: 9), and 3′-RTGNCCNCCNARNGCYTG-5′ (SEQ ID NO: 10) is illustrated (where N indicates inosine, R indicates G or A, and Y indicates C or T). The above-described primer sequences each correspond to an amino acid sequence QALGGH included in the zinc finger motifs of the above-described ZPT transcription factors.

Therefore, the stringent hybridization conditions which are applied to the present invention may also be used for PCR. In a representative example, the above-described degenerate primers (SEQ ID NOs: 9 and 10) may be used. In this case, the PCR reaction conditions may be the following: denaturation at 94° C. for 5 minutes; followed by 30 cycles of 30 seconds at 94° C., 30 seconds at 50° C. and 1 minute at 72° C.; and finally, incubation at 72° C. for 7 minutes.

PCR may be conducted based on the manufacturer's instruction for a commercially available kit and device, or a method well known to those skilled in the art. A method for preparing a gene library, a method for cloning a gene, and the like are also well known to those skilled in the art. For example, see Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd Ed. (Cold Spring Harbor Laboratory, 1989). The base sequence of a resultant gene may be determined with a nucleotide sequencing analysis method known in the art, or by a commercially available automatic sequencer.

In the present specification, “controlling the development of pollen” by a transcription factor representatively means that when the expression of this transcription factor is inhibited, a significant change in the form or functions of pollen is observed. Representatively, by inhibiting the expression of a gene encoding the transcription factor of the present invention, preferably about 75% or more, more preferably about 90% or more, and even more preferably about 95% or more of pollen cells are killed before being matured. When the amount of mRNA measured by a Northern blot method is about one tenth or less as compared to a wild-type control plant, the expression of a transcription factor is judged to be inhibited.

Whether or not the transcription factors encoded by genes isolated and identified by screening as above (i.e., ZPT2-5, ZPT3-1 and ZPT4-1, and the homologs thereof) control the development of pollen, can be confirmed by producing a transformed plant and observing the characteristics of the pollen of the plant in accordance with the disclosure of the present specification.

According to the present invention, DNA encoding a transcription factor which controls the development of pollen can be utilized to inhibit the expression of an endogenous gene having the same or homologous base sequence as that of the DNA in plant cells. Such a target endogenous gene is also a transcription factor which controls the development of pollen. According to the method of the present invention, plants are conferred male sterility by selectively inhibiting only the expression of an endogenous transcription factor, preferably without substantially inhibiting the expression of genes other than the endogenous transcription factor which controls the development of pollen.

In other words, plant cells to which the expression inhibiting technique of the present invention is applied are plant cells having an endogenous transcription factor which controls the development of pollen. The gene encoding this endogenous transcription factor is defined as a gene which hybridizes with DNA encoding the above-described ZPT2-5, ZPT3-1 or ZPT4-1, or a homolog thereof under stringent conditions. The definition of the “stringent conditions” is the same as that described in relation to specification of the homologs of ZPT2-5, ZPT3-1 and ZPT4-1. Plants capable of being conferred male sterility with the above-described method are preferably plants which are phylogenetically, closely related to Petunia from which the above-described ZPT genes are isolated, or plants from which genes encoding the above-described ZPT homologs are isolated, but the present invention is not intended to be limited to this. “Plants which are phylogenetically, closely related” means representatively plants categorized into the same order, preferably categorized into the same family, more preferably categorized into the same genus, and even more preferably categorized into the same species. Considering the fact that the development of pollen is essential for the reproduction of spermatophyte, it could be easily understood that transcription factors having the same or similar function to that of ZPT2-5, ZPT3-1 and ZPT4-1 may be widely present in other plants.

As a technique for suppressing the expression of an endogenous gene, cosuppression and antisense techniques may be utilized, representatively. As to cosuppression, when a recombinant gene is introduced into a plant cell, the expression of both the gene itself and an endogenous gene including a sequence homologous to part of that gene are suppressed. When cosuppression is utilized, an expression cassette according to the present invention includes DNA encoding a transcription factor or a fragment thereof in the form linked in a forward direction with respect to the promoter. After DNA encoding a transcription factor or a fragment thereof is introduced into a plant cell as an expression cassette, the DNA or fragment thereof can be transcribed in the sense direction under control of the promoter. Due to the action of the introduced DNA, it is possible to suppress the targeted gene expression. Cosuppression can be observed in some transformed plant individuals, but mostly, cosuppression does not occur sufficiently in other individuals. Therefore, typically, individuals in which gene expression is suppressed in an intended manner are screened with routine procedures.

Antisense means that when a recombinant gene is introduced into a plant cell, the transcribed product (mRNA) of the introduced gene forms a hybrid with the complementary sequence of the transcribed product (mRNA) of an endogenous gene so that the translation of a protein encoded by the endogenous gene is inhibited. When antisense is utilized, the expression cassette of the present invention includes DNA encoding a transcription factor or a fragment thereof in the form linked in a reverse direction with respect to the promoter. After DNA encoding a transcription factor or a fragment thereof is introduced into a plant cell as an expression cassette, the DNA or fragment thereof may be transcribed in the antisense direction under control of the promoter. Due to the action of the antisense transcripts, it is possible to suppress the expression of the targeted gene.

(Promoters Derived from ZPT3-1 and ZPT4-1 Genes)

A promoter useful in a method for producing a plant having a modified trait according to the fourth and fifth aspects of the invention is a promoter which includes any of (a′) DNA having a sequence from position 1 to position 2624 of a base sequence indicated by SEQ ID NO: 7; (a″) DNA having a sequence from position 1 to position 3631 of a base sequence indicated by SEQ ID NO: 8; and DNA having a part of the sequences (a′) or (a″) and which exhibits promoter activity specific to microspores. The above-described promoter of the present invention is preferably the promoter of (a′) or (a″), i.e., the promoter for the ZPT3-1 or ZPT4-1 gene.

A sequence having promoter activity specific to microspores, which is obtained by removing a sequence which is not essential for tissue-specific expression activity from the promoter regions for the ZPT3-1 and ZPT4-1 genes, falls within the scope of the present invention. Such a sequence can be obtained by conducting a promoter deletion experiment in accordance with a commonly used method. Briefly, a plasmid obtained by fusing various promoter region deletion mutants of the ZPT3-1 or ZPT4-1 gene (e.g., mutants obtained by deleting the promoter region from the 5′ upstream side of the ZPT3-1 or ZPT4-1 gene in various lengths), and an appropriate reporter gene (e.g., the GUS gene) can be used to measure the tissue-specific promoter activity of the deletion mutants, thereby identifying a region essential for the activity.

Once the region essential for the promoter activity is identified, it is possible that a sequence within or adjacent to the region is modified so that the magnitude of the expression activity of the promoter is increased. The thus-obtained variants also fall within the present invention as long as the variants exhibit promoter activity specific to microspores.

In the present invention, “exhibit promoter activity specific to microspores” means that the ability of a promoter to initiate the transcription of DNA to direct gene expression in a naturally-occurring plant or a plant to which the promoter is introduced as an expression cassette in which the promoter is linked to an arbitrary structural gene, is exhibited specifically in microspores. Here, “specific” means that the expression activity of a promoter is higher than in all the other tissues of the flower of the same plant (including tapetum layer, filament, style, capitulum, petal, calyx, and the like; note that the dehiscence tissue of the anther is excluded). The above-described specific promoter preferably has an expression activity in microspores, higher than the expression activity in all the other tissues of the flower and portions other than the flower of the same plant (roots, leaves, stems, and the like). More preferably, the specific promoter exhibits substantially no activity in all the other tissues of the flower and portions other than the flower of the same plant. “Exhibit the promoter activity specific to the dehiscence tissue of the anther” is defined in a manner similar to that described above. The magnitude of expression activity may be evaluated by comparing the expression level of a promoter in microspores with the expression level of the same promoter in other flower tissues in accordance with a commonly used method. The expression level of a promoter is typically determined by the production amount of the products of a gene expressed under control of the promoter.

The above-described method of the present invention utilizing a specific promoter is intended to modify a trait related to reproduction of a plant. “Modify” means that at least a portion of the reproductive organ of a post-transformation plant loses a function which existed in the pre-transformation plant (wild type or horticulture breed), acquires a function which did not exist in the pre-transformation plant, or has an increased or decreased level of particular function as compared to the pre-transformation plant. Such modification of a trait can be achieved as a result of the microspore-specific expression of any heterologous gene operatively linked to the promoter of the present invention under the control of the promoter in a transformed plant into which the gene has been introduced. It is well known that in a number of tissue-specific promoters, the tissue-specificity is conserved among species. Therefore, it is easily understood that the promoter of the present invention can be applied to a wide variety of plant species. The degree of trait modification may be evaluated by comparing the trait of a post-transformation plant with the trait of the pre-transformation plant. As a preferable trait to be modified, female sterility and self-incompatibility are illustrated, but such a trait is not limited to these.

For example, the promoter of the present invention can be obtained by screening the genomic library of a plant using known cDNA as a probe, and isolating an upstream sequence of a coding region from the corresponding genomic clone. As an example of cDNA, cDNA of the above-described transcription factors derived from Petunia, ZPT3-1 and ZPT4-1, are illustrated.

The promoter of the present invention is not limited to that isolated from the nature, but may include synthesized polynucleotides. For example, synthesized polynucleotides may be obtained by synthesizing or modifying the sequence of a promoter sequenced as described above or an active region thereof with a method well-known to those skilled in the art.

(Construction of Expression Cassette and Expression Vector)

DNA encoding the transcription factor of the present invention can be introduced into plant cells as an expression cassette, in which the DNA is operatively linked to an appropriate promoter using a method well known to those skilled in the art, with a known gene recombinant technique. Similarly, the microspore-specific promoter of the present invention can be introduced into plant cells as an expression cassette in which the promoter is operatively linked to a desired heterologous gene.

A “promoter” which can be linked to the above-described transcription factor means any promoter which expresses in plants, including any of a constitutive promoter, a tissue-specific promoter, and an inducible promoter.

“Constitutive promoter” refers to a promoter which causes a structural gene to be expressed at a certain level irrespective of stimuli inside or outside plant cells. When a heterologous gene is expressed in other tissues or organs of a plant and a plant is not given an undesired trait, use of a constitutive promoter is simple and preferable. As examples of such a constitutive promoter, 35S promoter (P35S) of cauliflower mosaic virus (CaMV), and the promoter for nopaline synthase (Tnos) are illustrated, but the constitutive promoter is not limited to these.

In the present invention, “tissue-specific promoter” refers to a promoter which causes a structural gene to be expressed specifically in at least microspores. Such a tissue-specific promoter includes the promoters derived from ZPT3-1 and ZPT4-1 genes of the present invention and, in addition, other known promoters having anther-specific expression activity. Therefore, use of an expression cassette of the naturally-occurring ZPT3-1 and ZPT4-1 genes comprising a microspore-specific promoter and a sequence encoding a transcription factor optionally combined with another regulatory element, falls within the present invention.

“Inducible promoter” refers to a promoter which causes a structural gene to be expressed in the presence of a particular stimulus, such as chemical agents, physical stress, and the like, and which does not exhibit expression activity in the absence of the stimulus. As an example of such an inducible promoter, a glutathione S-transferase (GST) promoter which can be induced by auxin (van der Kop, D. A. et al., Plant Mol. Biol., 39:979, 1999) is illustrated, but the inducible promoter is not limited to this.

In the present specification, the term “expression cassette” or “plant expression cassette” refers to a nucleic acid sequence including DNA encoding the transcription factor of the present invention and a plant expression promoter operatively (i.e., in such a manner that can control the expression of the DNA) linked to the DNA, and a nucleic acid sequence including the microspore-specific promoter of the present invention and a heterologous gene operatively (i.e., in-frame) linked to the promoter.

“Heterologous gene” which may be linked to the above-described microspore-specific promoter refers to any of endogenous genes of Petunia other than the ZPT3-1 and ZPT4-1 gene, endogenous genes in a plant other than Petunia, or genes exogenous to plants (e.g., genes derived from animals, insects, bacteria, and fungi), where the expression of products of such a gene are desired in microspores. A preferable example of such a heterologous gene in the present invention is a gene which encodes a cytotoxic gene product and whose expression inhibits the development of pollen. As a specific example of such a gene, the barnase gene (Beals, T. P. and Goldberg, R. B., Plant Cell, 9:1527, 1997) is illustrated, but the present invention is not limited to this.

“Plant expression vector” refers to a nucleic acid sequence including an expression cassette and, in addition, various regulatory elements linked to the cassette in such a manner that the regulatory elements can be operated in host plant cells. Preferably, such a plant expression vector may include a terminator, a drug-resistant gene, and an enhancer. It is well known matter to those skilled in the art that the types of plant expression vectors and the types of regulatory elements used may be varied depending on host cells. Plant expression vectors used in the present invention may further have a T-DNA region. The T-DNA region increases the efficiency of gene introduction, particularly when Agrobacterium is used to transform a plant.

“Terminator” is a sequence which is located downstream of a region encoding a protein of a gene and which is involved in the termination of transcription when DNA is transcribed in to mRNA, and the addition of a poly A sequence. It is known that a terminator contributes to the stability of mRNA, and has an influence on the amount of gene expression. As examples of such a terminator, the terminator for the nopaline synthase gene (Tnos), and the 35S terminator of cauliflower mosaic virus (CaMV) are illustrated, but the terminator is not limited to these.

“Drug-resistant gene” is desirably one that facilitates the selection of transformed plants. The neomycin phosphotransferase II (NPTII) gene for conferring kanamycin resistance, and the hygromycin phosphotransferase gene for conferring hygromycin resistance may be preferably used, but the drug-resistant gene is not limited to these.

The plant expression vector of the present invention may be prepared using a gene recombinant technique well known to those skilled in the art. A plant expression vector is constructed, for example, preferably using pBI-type vectors or pUC-type vectors, but the plant expression vector is not limited to these.

(Production of Transformed Plant)

The thus-constructed expression cassette, or an expression vector including the expression cassette, may be introduced into desired plant cells using a known gene recombinant technique. The introduced expression cassette is present to be integrated into DNA in a plant cell. It should be noted that DNA in a plant cell includes not only chromosome but also DNA included in various organelles included in a plant cell (e.g., a mitochondria, and a chloroplast).

In the present specification, the term “plant” includes any of monocotyledons and dicotyledons. Preferable plants are dicotyledons. Dicotyledons include any of Archichlamiidae and Sympetalidae. A preferable subclass is Sympetalidae. Sympetalidae includes any of Gentianales, Solanales, Lamiales, Callitrichales, Plantaginales, Campanulales, Scrophulariales, Rubiales, Dipsacales, and Asterales. A preferable order is Solanales. Solanales includes any of Solanaceae, Hydrophyllaceae, Polemoniaceae, Cuscutaceae, and Convolvulaceae. A preferable family is Solanaceae. Solanaceae includes Petunia, Datura, Nicotiana, Solanum, Lycopersicon, Capsicum, Physalis, Lycium, and the like. Preferable genera are Petunia, Datura, and Nicotiana, and more preferably Petunia. The genus Petunia includes the following species: P. hybrida, P. axillaris, P. inflata, P. violacea, and the like. A preferable species is P. hybrida. “Plant” means phanerogamic plants and seed obtained from the plants unless otherwise specified.

As examples of “plant cells”, cells in each tissue of plant organs, such as flowers, leaves, roots, and the like, callus, and suspension cultured cells are illustrated.

For the purpose of introduction of a plant expression vector into a plant cell, a method well known to those skilled in the art, such as an indirect method using Agrobacterium, and a method for directly introducing into cells, can be used. As such an indirect method using Agrobacterium, for example, a method of Nagel et al. (FEMS Microbiol. Lett., 67:325 (1990)) may be used. In this method, initially, Agrobacterium is transformed with a plant expression vector (e.g., by electroporation), and then the transformed Agrobacterium is introduced into a plant cell with a well-known method, such as a leaf disk method and the like. As a method for directly introducing a plant expression vector into a cell, an electroporation method, particle gun, a calcium phosphate method, a polyethylene glycol method, and the like are illustrated. These methods are well known in the art. A method suitable for a plant to be transformed can be appropriately selected by those skilled in the art.

Cells into which a plant expression vector has been introduced are screened for drug resistance, such as kanamycin resistance and the like, for example. A selected cell may be regenerated to a plant using a commonly used method.

Whether or not an introduced plant expression vector is operative in a regenerated plant can be confirmed with a technique well-known to those skilled in the art. For example, in the case where suppression of the expression of an endogenous gene is intended, such confirmation can be conducted by measuring the level of transcription with Northern blot analysis. In this manner, a desired transformed plant in which the expression of an endogenous transcription factor is suppressed can be selected. For the purpose of the expression of a heterologous gene using a tissue-specific promoter, the expression of the heterologous gene can be confirmed usually by Northern blot analysis using RNA extracted from a target tissue as a sample. The procedures of this analysis method are well known to those skilled in the art.

Whether or not the expression of an endogenous transcription factor is suppressed in accordance with the method of the present invention so that pollen fertility is reduced can be confirmed, for example, by observing the form of the pollen of a plant, which is transformed by an expression vector including DNA encoding a transcription factor, with a microscope optionally after histochemically staining.

Whether or not a promoter is expressed specifically in a microspore in accordance with the method of the present invention can be confirmed by, for example, histochemically staining flower tissues including the anther in a plant transformed with an expression vector, in which a promoter is operatively linked to the GUS gene, by a commonly used method to detect the distribution of GUS activity.

EXAMPLES

Hereinafter, the present invention will be described based on examples. The scope of the present invention is not limited to the examples only. Restriction enzymes, plasmids, and the like used in the examples are available from commercial sources.

Example 1 Construction of Plant Expression Vector Including Polynucleotide Encoding ZPT Transcription Factors

Out of the previously reported anther-specific ZF genes (Kobayashi et al., above), cDNAs of PEThy ZPT2-5 (ZPT2-5), PEThy ZPT3-1 (ZPT3-1), and PEThy ZPT4-1 (ZPT4-1) were each linked downstream of the 35S promoter of the cauliflower mosaic virus to prepare a plant expression vector. This preparation will be specifically described below.

Example 1-1

DNA fragments including the cauliflower mosaic virus 35S promoter (HindIII-XbaI fragment) and DNA fragments including the NOS terminator (SacI-EcoRI fragment) in plasmid pBI221 (purchased from CLONTECH Laboratories Inc.) were successively inserted into the multi-cloning site of plasmid pUCAP (van Engelen, F. A. et al., Transgenic Res., 4:288, 1995) to prepare pUCAP35S. A pBluescript vector including cDNA of ZPT2-5 was cleaved at KpnI and SacI sites (either is a site within the vector), and inserted between KpnI and SacI sites of the above-described pUCAP35S. Further, this recombinant plasmid was cleaved with EcoRI and HindIII, and a DNA fragment encoding ZPT2-5 was inserted between EcoRI and HindIII sites of binary vector pBINPLUS (van Engelen, F. A. et al., above). As shown in FIG. 4( a), the constructed ZPT2-5 gene comprises the 35S promoter region (P35S; 0.9 kb) of cauliflower mosaic virus (CaMV), a polynucleotide (ZPT2-5; about 0.8 kb) encoding ZPT2-5 of the present invention, and the terminator region of nopaline synthase (Tnos; 0.3 kb). Pnos in FIG. 4 indicates the promoter region of nopaline synthase, and NPTII indicates the neomycin phosphotransferase II gene.

Example 1-2

A pBluescript vector including cDNA of ZPT3-1 was cleaved at KpnI site and SacI site (either is a site within the vector), and inserted between KpnI and SacI sites of pUCAP35S. Further, this recombinant plasmid was cleaved with EcoRI and HindIII, and a DNA fragment encoding ZPT3-1 was introduced between EcoRI and HindIII sites of binary vector pBINPLUS. As is apparent from FIG. 4( b), the constructed ZPT3-1 gene comprises the 35S promoter region (P35S; 0.9 kb) of cauliflower mosaic virus (CaMV), a polynucleotide (ZPT3-1; about 1.7 kb) encoding ZPT3-1 of the present invention, and the terminator region (Tnos; 0.3 kb) of nopaline synthase.

Example 1-3

A pBluescript vector including cDNA of ZPT4-1 was cleaved at KpnI site and SacI site (either is a site within the vector), and inserted between KpnI and SacI sites of the above-described pUCAP35S. Further, this recombinant plasmid was cleaved with EcoRI and HindIII, and a DNA fragment encoding ZPT4-1 was introduced between EcoRI and HindIII sites of binary vector pBINPLUS. As is apparent from FIG. 4( c), the constructed ZPT4-1 gene comprises the 35S promoter region (P35S; 0.9 kb) of cauliflower mosaic virus (CaMV), a polynucleotide (ZPT4-1; about 2.0 kb) encoding ZPT4-1 of the present invention, and the terminator region (Tnos; 0.3 kb) of nopaline synthase.

Example 2 Isolation of ZPT3-1 and ZPT4-1 Promoter Regions and Linkage to GUS Reporter Gene

cDNAs of ZPT3-1 and ZPT4-1 were used as probes to isolate corresponding genomic clones from the genome DNA library of Petunia. DNA fragments (promoter region; about 2.7 kb and about 3.6 kb) upstream of the transcription initiation site were subcloned. Each DNA fragment was linked upstream of the GUS reporter gene and cloned into a binary vector. This preparation will be specifically described below.

Example 2-1

cDNA of ZPT3-1 was labeled with [α-³²P]dCTP using a commonly used random priming method (Sambrook et al., above) to prepare a radiolabeled probe. With this probe, a genomic library of Petunia (Petunia hybrida var. Mitchell) prepared within EMBL3 vector (manufactured by Stratagene) was screened. A genome DNA fragment (PstI-SacI) of about 2.7 kb including the upstream region of the gene from the resultant clone was subcloned at PstI-SacI site of pBluescriptSK vector (pBS-ZPT3-1-PS), followed by sequencing (FIG. 5). Next, this plasmid was used as a template to conduct PCR using a primer including a SalI recognition sequence (3′-TATGGAGCTCGTCGACAG TTGATGGTTCATTTTTCTGGCTATTGTC-5′; SEQ ID NO: 11) and a commercially available M13–20 primer, so that SalI site was introduced immediately downstream of the initiation site of translation of the ZPT3-1 protein (base position: 2661). Thereafter, a DNA fragment cleaved with PstI and SalI was inserted upstream of the GUS coding region of pUCAPGUSNT (pUCAP-ZPT3-1-GUSNT). Therefore, the ZPT3-1 gene was connected to the GUS coding region in frame at a region near the N terminus of the coding region of the ZPT3-1 gene. Further, a DNA fragment obtained by cleaving pUCAP-ZPT3-1-GUSNT with AscI and PacI (including the ZPT3-1 promoter, the GUS coding region and the NOS terminator) was inserted into pBINPLUS vector to obtain pBIN-ZPT3-1-GUS (FIG. 7( a)).

Example 2-2

As to ZPT4-1, similarly, genomic DNA was isolated, and a DNA fragment (EcoRI—EcoRI) of about 3.6 kb including an upstream region of the ZPT4-1 gene was subcloned at the EcoRI—EcoRI site of pBluescriptSK vector (pBS-ZPT4-1-EE), followed by sequencing (FIG. 6). This plasmid was used as a template to conduct PCR using a primer including a BamHI recognition sequence (3′-CATGGATATAGGATCCTATATC-5′; SEQ ID NO: 12) and M13–20 primer, so that BamHI site was introduced immediately downstream of the initiation site of translation of the ZPT4-1 protein (base position: 3641). Thereafter, a DNA fragment cleaved with EcoRI and BamHI was inserted upstream of the GUS coding region of pUCAPGUSNT (pUCAP-ZPT4-1-GUSNT). Therefore, the ZPT4-1 gene was connected to the GUS coding region in frame at a region near the N terminus of the coding region of the ZPT4-1 gene. Further, a DNA fragment (AscI-PacI) was inserted into pBINPLUS vector to obtain pBIN-ZPT4-1-GUS in a manner similar to that described above (FIG. 7( b)).

Example 3 Introduction of Each Fusion Gene into Petunia Cells

Each of the above-described expression vectors was introduced via Agrobacterium into Petunia (Petunia hybrida var. Mitchell) with the following procedures.

(1) Agrobacterium tumefaciens LBA4404 strain (purchased from CLONTECH Laboratories Inc.) was cultured at 28° C. in L medium containing 250 mg/ml of streptomycin and 50 mg/ml of rifampicin. Cell suspension was prepared in accordance with the method of Nagel et al. (1990) (above). The plasmid vector constructed in Examples 1 and 2 were introduced into the above-described strain by electroporation.

(2) A polynucleotide encoding each fusion gene was introduced into Petunia cells using the following method: the Agrobacterium tumefaciens LBA4404 strain obtained in the above-described (1) was shake-cultured (28° C., 200 rpm) in YEB medium (DNA Cloning, Vol. 2, page 78, Glover D. M. Ed., IRL Press, 1985). The resultant culture was diluted with sterilized water by a factor of 20, and cocultured with leaf pieces of Petunia (Petunia hybrida var. Mitchell). After 2 to 3 days, the above-described bacterium was removed in medium containing antibiotics. The Petunia cells were subcultured with selection medium every two weeks. The transformed Petunia cells were selected based on the presence or absence of kanamycin resistance due to the expression of the NPTII gene derived from pBINPLUS which had been introduced along with the above-described five fusion genes. The selected cells were induced into callus with a commonly used method. The callus was redifferentiated into a plant (Jorgensen R. A. et al., Plant Mol. Biol., 31:957, 1996).

Example 4 Phenotype of Transformed Petunia into which ZPT Genes are Introduced

The transformants obtained by introducing the vector of Example 1 were used to observe change in the form of pollen in association with the control of the expression of ZPT2-5, ZPT3-1 and ZPT4-1, so that the influence of the introduced cDNA of these ZPT genes on plants were studied. This study will be described below in detail.

Example 4-1

From transformants (14 individuals) into which cDNA of ZPT2-5 had been introduced under the control of a 35S promoter, individuals (3 individuals) in which gene expression was suppressed by cosuppression were selected by Northern blot analysis (note that over expression of the ZPT2-5 gene introduced was observed in four individuals out of the 14 individuals). The conditions of the Northern blot analysis were the following: hybridization was conducted in a solution containing 7% SDS, 50% formamide, 5×SSC, 2% blocking reagent (manufactured by Boehringer Mannheim), 50 mM sodium phosphate buffer (pH 7.0), 0.1% sodium lauryl sarcosine, 50 μg/ml of yeast tRNA, and ³²P-labeled probe DNA (1×10⁷ cpm) at 68° C. for 16 hours, followed by washing with 2×SSC/0.1% SDS at 68° C. for 30 minutes.

In the above-described three cosuppression transformants, the following phenotypes were observed (FIG. 8).

In the meiosis process which occurs immediately before the tetrad stage, in the case of normal (wild type) Petunia, chromatin is condensed into thin thread-like structures (prophase I: leptotene), and synapsis of homologous chromosomes occurs (prophase I: zygotene). Thereafter, in metaphase I, chromosome tetrad align along the equatorial plane of the cells, and thereafter the homologous chromosomes are equally separated to the opposite poles of the cells by the spindle apparatus. In the transformant having cosuppression of the ZPT2-5 gene, the separation of the chromosomes to the poles proceeded while chromosome tetrad did not align along the equatorial plane in metaphase I. The division of the chromosomes to the poles was significantly unbalanced.

In the normal process of meiosis, after the above-described first separation of the chromosomes, second separation of the chromosomes forms four haploid groups. Thereafter, separation of cytoplasm occurs. In the case of the above-described transformant having cosuppression, separation of cytoplasm and cell division occurred immediately after the first separation of chromosomes. This unbalanced cell division occurred not only at a single time but also further repeated at least two times, so that 8 microspore cells were formed at the most. Due to the unbalanced separation of chromosomes, the number of chromosomes included in the microspore cells was unequal and, in addition, the size of the cells was significantly unequal. As a result, during the stage corresponding to the tetrad stage of normal Petunia, a more number of microspores (8 or less) than normal were formed in these transformants (FIG. 8( f); a photograph of pollen cells of the ZPT2-5 cosuppression transformant in the bud having a size of 6 mm. Further, FIG. 9( b); see a photograph of pollen cells of the transformant in the tetrad stage).

In the cosuppression transformants, a part of the microspores (10–20%) still continued to develop, but most microspore cells burst before a callose layer enveloping the microspore was degraded. In this stage, the microspores which did not burst and survived were in the abnormal form of substantially a hexahedron, which was clearly different from the tetrahedron form of normal microspores. Thereafter, the abnormal-form microspores became binuclear due to seemingly normal mitosis to form pollen grains. However, most of these pollen grains lost fertility. Specifically, when the pollen grains of these transformants were placed on the pistil of normal Petunia, no or few seeds were formed from pollen of the three strains exhibiting cosuppression (10% or less, i.e., the number of seeds produced by one Petunia is 10% of control as the average of about 10 flowers). For pollen from three transformant strains without cosuppression, normal seed formation was confirmed similar to wild type control plants.

The above-described cosuppression transformant also exhibited abnormality in formation of female gametophyte, and female fertility was reduced to 25–35% of that of normal individuals. Specifically, the development of an ovule (female gametophyte) was seemingly normal, but when wild type pollen was used for pollination, the majority of ovules could not be fertilized and even fertilized ovules exhibited abnormality in the subsequent developement, so that most ovules aborted. In this case, the transformants without cosuppression formed normal female gametophytes similar to wild type control plants.

Example 4-2

cDNA of ZPT3-1 was introduced under the control of the 35S promoter. As a result, a trait change similar to when the ZPT2-5 gene was introduced appeared in three individuals out of 15 individuals (FIG. 9). Specifically, in these transformants, substantially the same abnormality as that of ZPT2-5 were observed in the process of meiosis. The number of cells which developed up to the microspore stage was very small, and surviving microspores exhibited morphological abnormality (hexahedron). Further, matured pollen grains lost fertility. However, unlike ZPT2-5, the female fertility of these individuals was not affected.

Gene expression was analyzed with the Northern blot method under the same conditions as those in Example 4-1. As a result, in individuals into which the ZPT3-1 gene was introduced, gene expression was suppressed both for ZPT3-1 and ZPT4-1. Both genes share a high level of structural similarity. Specifically, the homology of the base sequence in the entire coding region is 37%. When the second ZF region of ZPT3-1 and the third ZF region of ZPT4-1, and the third ZF region of ZPT3-1 and the fourth ZF region of ZPT4-1, including neighboring sequences, are respectively compared with each other at the base sequence level in such a manner that the homology value is maximized, the average of the homology is 86% (the comparison of the sequences was conducted using the Clustal V program). Therefore, it is highly likely that the above-described expression suppressing phenomenon is caused by the introduction of one gene leading to the suppression of the expression of two genes (cosuppression). This suggests that the functions of these two genes overlap, and is consistent in that by the introduction of either gene, a common change in a phenotype could be observed.

Example 4-3

cDNA of ZPT4-1 was introduced under the control of the 35S promoter. As a result, a trait change similar to when the ZPT2-5 gene was introduced appeared in two individuals out of 13 individuals. Specifically, in these transformants, substantially the same abnormality as that of ZPT2-5 were observed in the process of meiosis. The number of cells which developed up to the microspore stage was very small, and surviving microspores exhibited morphological abnormality (hexahedron). Further, most matured pollen grains lost fertility. However, similar to ZPT3-1, the female fertility of these individuals was not affected. For the above-described reasons, in this example, it is also highly likely that gene expression was suppressed for both ZPT3-1 and ZPT4-1 (cosuppression).

As described above, by introducing a gene encoding ZPT2-5, ZPT3-1 or ZPT4-1, the development of pollen can be inhibited and the fertility can be eliminated with excellent efficiency (99% or more for ZPT3-1, and 90% or more for ZPT2-5 and ZPT4-1). The introduction of these genes may be useful for a selective trait transformation technique since the effects of the genes are specific to pollen (pollen and female gametophyte in the case of ZPT2-5) and the other traits of plants are not affected.

Example 5 Tissue Specificity of Promoter Activity of ZPT3-1 and ZPT4-1

The tissue-specific promoter activity of the above-described DNA fragments was detected by histochemical staining with GUS activity using the transformants obtained by introducing the vector in Example 2. This will be described below in detail.

Example 5-1

The flowers of the transformants obtained by introducing a fusion gene of the upstream region of the ZPT3-1 gene with GUS were used to study the distribution of GUS activity using X-GUS as a substrate (Gallagher, S. R. Ed., GUS protocols: using the GUS gene as a reporter of gene expression, Academic Press, Inc., pp. 103–114, 1992). As a result, GUS activity was detected specifically in microspores in the uninucleate stage (FIGS. 10( a) through (c)).

Example 5-2

The flowers of the transformants obtained by introducing a fusion gene of the upstream region of the ZPT4-1 gene with GUS were used to study the distribution of GUS activity in a manner similar to that described above. As a result, GUS activity was observed specifically in microspores and the dehiscence tissue of anthers from the uninucleate stage to the binucleate stage (FIG. 10( d) through (f); the dehiscence tissue of anthers was indicated by an arrow in FIGS. 10( e) and (f)).

As described above, the promoters for the ZPT3-1 and ZPT4-1 genes exhibit activity specifically in microspores in the uninucleate stage (ZPT3-1) and microspores from the uninucleate stage to the binucleate stage (ZPT4-1), respectively. The promoter for the ZPT4-1 gene also exhibits activity specifically in the dehiscence tissue of anthers from the uninucleate stage to the binucleate stage.

Microspores are precursor cells which will be subsequently matured to form pollen grains. Therefore, these promoters are useful as a tool for detailed research on the development of pollen. Further, these promoters or active fragments thereof can be used to cause a cytotoxic gene or the like to be expressed specifically in microspores to abort pollen cells or eliminate the functions thereof, whereby the development of pollen can be directly and efficiently controlled.

INDUSTRIAL APPLICABILITY

The method of the present invention utilizing DNA encoding transcription factors derived from the ZPT2-5, ZPT3-1 and ZPT4-1 genes, and promoters derived from the ZPT3-1 and ZPT4-1 genes is useful as a technique for selectively modifying the trait of a plant using a genetic engineering method, particularly a technique for conferring male sterility. 

1. A method for producing a male sterile plant of the genus Petunia, comprising the steps of: providing a plant expression cassette comprising a nucleic acid consisting of a DNA which encodes a zinc finger transcription factor, wherein the DNA has the base sequence from position 1 to position 777 of the base sequence indicated by SEQ ID NO:1 providing Petunia plant cells having an endogenous transcription factor controlling the development of pollen, wherein the gene encoding the endogenous transcription factor hybridizes under stringent conditions to the nucleic acid; introducing the expression cassette into the plant cells; regenerating the plant cells, into which the expression cassette has been introduced, into plants to produce regenerated plants, and screening the regenerated plants for one in which the nucleic acid is expressed so that expression of the endogenous transcription factor is suppressed.
 2. The method according to claim 1, wherein the nucleic acid is linked in a forward direction with respect to the promoter, and is transcribed in the sense direction in cells of the plant.
 3. The method according to claim 1, wherein the nucleic acid is linked in a reverse direction with respect to the promoter, and is transcribed in an antisense direction in cells of the plant.
 4. The method according to claim 1, wherein the expression cassette is incorporated into a plant expression vector.
 5. A male sterile plant produced by the method according to any one of claim 2, 3 or
 4. 